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A white dwarf with a close orbit around a black hole

Artist’s impression of the X9 system with the view from Chandra X-ray Observatory in the inset (Image X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes a research on what appears to be a binary system including a star with the closest orbit around a black hole. A team of astronomers used NASA’s Chandra X-ray Observatory and NuSTAR space Telescope and the Australia Telescope Compact Array (ATCA) to observe this system called X9 concluding that the star, a white dwarf, takes 28 minutes to orbit the black hole.

The X9 system is located in the 47 Tucanae globular cluster, 14,800 light years from Earth. It’s a system known for some time but astronomers thought that the white dwarf was orbiting around another star. In 2015, however, a team of astronomers used observations made at radio frequencies with the ATCA showing that the white dwarf’s companion had features which indicate that it’s probably a black hole.

In essence, the X9 system is formed by the remains of two stars one of which was probably similar to the Sun, with a mass not very high so that at the end of its normal life what’s left became a white dwarf and another one that was much more massive, with the result that at the end of its life what’s left became a black hole. It’s not the first time that such couples are found but among the ones known this is the couple in which the two companions are the closest.

The observations with the Chandra X-ray Observatory were particularly useful because they showed that the X9 system varies its brilliance at X-rays the same way every 28 minutes. This suggests that this is the time taken by the white dwarf to complete one orbit around the black hole. The collected data also show large amounts of oxygen in the system and white dwarfs are rich in this element, therefore a confirmation of its nature. Its distance from the black hole is about two and a half times that of the Moon from the Earth, really very little.

Arash Bahramian of the University of Alberta, Canada, and the Michigan State University, USA, explained that the white dwarf is so close to the black hole that materials are sucked from it and thrown in the disk that surrounds the black hole. Although not in danger of being sucked directly by the black hole, the white dwarf will keep on losing mass and could even end up fading away into the distant future.

The main question for the researchers concerns the way in which the white dwarf ended up so close to the black hole. One possibility is that the black hole hit a red giant and gas that formed the star’s outer regions was dispersed and from its core the white dwarf emerged. Another possibility is that the white dwarf’s companion is actually a neutron star but it would be a pulsar that rotates on its axis thousands of times per second so it should have an extreme variability at X-ray and radio emissions, which was not detected.

The X9 system is also interesting as a target for future observations of gravitational waves. The current instruments such as LIGO can’t detect them but projects in development such as LISA, which will work in space, might be sensitive enough and allow to add this revolutionary type of observations of X9 to those of its electromagnetic emissions.